This invention pertains to steel mills and, more particularly, to a process for protecting bearings in steel mills.
In steel mills, hot molten steel is formed into slabs in a hot steel slab caster. In slab casters, molten steel enters a formation chamber. One or more steel slabs emerge from the formation chamber with a thin skin of solidified steel holding them together. The steel emerging from the formation chamber can be in the form of a series of discrete slabs or, alternatively, as one unbroken slab which is cut into discrete slabs at the far end of the slab caster. This latter process is characteristic of the more modern facilities and is usually referred to as a continuous caster. Steel slabs can vary in width and thickness depending on the particular steel mill, but a standard width for a single strand of steel on a continuous caster is about six feet with a thickness of 9-12 inches. Steel slabs, once cut, are typically about 25 feet long.
In order to convey the steel slab from the formation chamber, the slab is supported by a series of rotatable caster rollers. Each of these caster rollers has a bushing or bearing, usually a tapered roller bearing, at each end which allows the caster roller to turn. The line or lines of caster rollers in steel mills can be as long as three miles with a caster roller every two feet. Such a line or lines can use three million pounds of grease per year. Because the caster rollers are not much wider than the steel slab it supports, the steel slab typically comes within only a very few inches of the bearings. The bearings and grease used to lubricate those bearings experience very high thermal stress, with the steel slab surface often irradiating at temperatures of 1,500.degree. F. to 2,000.degree. F. Also, steel slabs exert a large force on each caster roller due to the heavy weight of the slabs causing high loading pressures on the bearings and bearing grease.
High performance greases are important to minimize failure of the caster bearings. Such bearing failures will cause the caster to stop rotating under the progressing steel slab. If this occurs, the dragging force between the slab surface and the nonrotating caster roller can rupture the slab skin causing a breakout which can endanger operating personnel, damage property and interrupt steel mill operations and production.
For example, when the hot steel slab moves along the series of caster rollers, the slab is quickly quenched and cooled to strengthen and thicken the solid skin of the slab. If quenching is not done properly, the tenuous skin can rupture causing molten steel to flow out onto the caster rollers, bearing housings, and eventually the plant floor. Such an occurrence (breakout) is very costly in terms of plant downtime and maintenance cost. To minimize breakouts, rapid quenching, cooling and strengthening of the skin is accomplished by high velocity water spray from all directions. The spray velocity can be as high as 1,000 gallons per minute. With such water spray force, even well sealed bearings will not totally exclude water. Therefore, the bearing grease will experience water contamination with a physical force that tends to wash (flush) the grease out of the bearings.
Another problem associated with conventional steel mill greases which is becoming of great concern is the increasing number and intensity of grease fires. Grease fires can occur from hot molten metal, from acetylene torches during periodic maintenance, and from other sources of ignition. Grease fires can be costly in terms of loss of equipment, operational downtime, and loss of life. It is highly desirable to have a high performance steel mill grease which also reduces the occurrence of grease fires.
Once formed and sufficiently cooled, steel slabs can be fabricated into other more commercially useful forms in process mills, such as hot strip mills, cold strip mills, billet mills, plate mills, and rod mills. Although the lubricant environment for process mills are not as severe as slab casters, grease specifications are quite stringent because of the high operating temperature and extreme pressure, antiwear requirements. Grease mills which purify, form, and process other metals such as aluminum encounter many similar problems as steel mill greases.
Preferably, the grease used to lubricate the bearings of hot slab casters should: (a) reduce wear and friction; (b) prevent rusting even in presence of water sprays; (c) be passive, non-corrosive, and unreactive with the bearing material; (d) resist being displaced by high velocity water sprays; and (e) maintain the integrity of its chemical composition and resulting performance properties under operating conditions near thermal sources which irradiate at temperatures of 1,500.degree. F. to 2,000.degree. F.
In order to enhance the safety, health, and welfare of operating personnel, greases used in steel mills should be non-toxic, reduce the incidence of grease fires, and be of a safe composition. Materials known to be serious skin irritants, carcenogenic, and mutogenic should be avoided in steel mill greases.
Grease used to lubricate tapered roller bearings of slab casters and process mills in steel mills should desirably have good adherence properties as well as resist displacement by water spray. The grease should retain these properties during use without exhibiting any adverse effects such as lacquer deposition on the tapered roller bearing parts due to high temperature oxidation, thermal breakdown, and polymerization of the lubricating grease. Such lacquering problems have been a common occurrence in hot slab casters especially where aluminum complex and lithium complex thickened greases have been used. When such lacquering becomes severe enough, the results are similar to rusting: the caster bearing fails and a breakout can occur.
Since hot slab caster bearing grease may be used in other applications in the steel mill, additional properties such as good elastomer compatibility and protection against other types of wear such as fretting wear is desirable. Also, many steel manufacturers prefer a grease which would work well in slab casters and in process mills, thereby allowing a multi-use consolidation of lubricants and a reduction in lubricant inventory.
Over the years, a variety of greases and processes have been suggested for use in steel mills and other applications. Typifying such greases and processes are those found in U.S. Pat. Nos. 2,964,475, 2,967,151, 3,344,065, 3,843,528, 3,846,314, 3,920,571, 4,107,058, 4,305,831, 4,431,552, 4,440,658, 4,514,312, and Re. 31,611. These prior art greases and processes have met with varying degrees of success. None of these prior art greases and processes, however, have been successful in simultaneously providing all the above stated properties at the high performance levels required in steel mills.
It is, therefore, desirable to provide an improved process which overcomes many, if not all, of the preceding problems.